Subtitle Timing Shifter

Subtitle sync breaks in one of two ways. The first is a constant offset: every cue fires at the same fixed distance from where it should — 2 seconds late, 0.4 seconds early, uniformly across the entire file. This happens when a video is re-encoded with an encoder that handles the start timestamp differently, when a pre-roll intro is trimmed from the front of the video but the SRT was built against the original, or when a subtitle file produced for one version of a video is applied to a slightly different cut. The second is frame-rate drift: a gradually increasing error where early cues are slightly off and late cues are significantly off, caused by a frame-rate mismatch between the video the SRT was built against and the one it is being used with. The AT USE Subtitle Timing Shifter fixes the first kind: constant offsets. It does not fix drift.

The tool takes a shift value in seconds — positive shifts every cue later, negative shifts every cue earlier — and rewrites every timestamp in the file. Enter -2 to fix subtitles that fire 2 seconds late. Enter 3 to fix a file produced before a 3-second intro was added. The shift accepts three decimal places: -0.317 and 2.500 are both valid. This matters for sub-second sync problems, which are common when auto-captioning services produce an SRT that is a few frames ahead of the actual audio. All formats that carry timestamp data — SRT, WebVTT, ASS/SSA, SBV — are supported. Processing runs entirely in your browser; your subtitle files never leave your device.

Partial-file shifting

The "Start from" field shifts only the cues that appear after a specified time in the video. Use this when a video edit inserted a new segment mid-file: cues before the insertion point are correct, and only the cues after it need to shift forward. Set the offset to match the inserted segment duration and set "Start from" to the time just after the insertion point. Cues before that timestamp are written back unchanged; only cues at or after the threshold are shifted.

What the tool changes — and what it does not

Only the start and end timestamp of each cue is modified. The cue text, sequence numbers (SRT), actor labels (ASS), styling tags, and all format-specific metadata are written back exactly as they were. The relative duration of each cue is preserved — a cue that spans 1.8 seconds still spans 1.8 seconds in the output. The only change is the absolute position of each cue in the video timeline.

Round-trip accuracy

Because the tool stores timestamps at millisecond precision without rounding (SRT timestamps resolve to milliseconds; WebVTT timestamps resolve to milliseconds), a shift-and-reverse operation is lossless. Shift a file by -2.5, then shift the output by 2.5, and the resulting timestamps are identical to the original. This makes it safe to experiment with shift values until the sync is correct without accumulating rounding error from repeated adjustments.

What to do when the offset does not feel constant

If the first subtitle is 1 second late but the last subtitle is 4 seconds late, the problem is frame-rate drift, not a constant offset. A single shift value will not fix this — applying -2.5 makes the middle correct but the start and end wrong. Frame-rate drift requires a subtitle editor that can apply a proportional time stretch (multiplying every timestamp by a correction factor). Aegisub is the standard desktop tool for this; the approach is to calculate the ratio of the source and target frame rates and apply it as a "linear time stretch" operation.